1. Field of the Invention
The present invention relates to a method of manufacturing a nozzle plate, a liquid droplet ejection head and an image forming apparatus, and more particularly to a method of manufacturing a nozzle plate in which nozzles for ejecting droplets of liquid are formed.
2. Description of the Related Art
The print head of an inkjet type image forming apparatus has a plurality of nozzles formed in a nozzle plate, which constitutes an ejection surface that opposes the recording medium. The shape of the nozzles which eject ink droplets onto the recording medium readily affects the size and the ejection speed, and the like, of the ink droplets, and therefore, the nozzles must be processed to a high degree of accuracy.
A processing method using electroforming (hereinafter referred to as “electroforming method”) is known as a method of manufacturing a nozzle plate of this kind. A characteristic feature of the electroforming method is that it allows nozzle plates to be manufactured at low cost, compared to a processing method using a laser beam or a processing method using a press.
FIGS. 11A to 11C show a first related method of manufacturing a nozzle plate based on an electroforming method in the related art. Firstly, as shown in FIG. 11A, a photosensitive resin (hereinafter referred to as “resist”) 202 which is photocurable is formed and patterned on a metal substrate 200. The plan shape of the resist 202 is circular, and the center thereof substantially coincides with the center of a nozzle to be formed. Next, as shown in FIG. 11B, a metal layer 204 of nickel plating, or the like, for example, is grown and formed on the metal substrate 200 by means of an electroforming method. When the thickness of the metal layer 204 exceeds the thickness of the resist 202, the metal layer 204 is grown in such a manner that the metal layer 204 gradually covers the resist 202 from the periphery thereof. Then, a recess portion 206 having an internal wall with a curved cross-sectional shape is formed. In other words, the metal layer 204 is formed to overhang the resist 202. Finally, as shown in FIG. 11C, the metal substrate 200 and the resist 202 are peeled away from the metal layer 204, and the metal layer 204 corresponding to a nozzle plate 260 is thus obtained. Nozzles (through holes) 251 each having a curved cross-section corresponding to the recess portions 206 in FIG. 11B are formed in the nozzle plate 260.
FIGS. 12A and 12B show a second related method of manufacturing a nozzle plate based on an electroforming method in the related art. Firstly, a patterned resist 302 is formed in a substantially circular cylinder shape on a metal substrate 300, and a metal layer 304 is grown on the metal substrate 300 to a height lower than the height of the resist 302, as shown in FIG. 12A. Then, the metal substrate 300 and the resist 302 are peeled away from the metal layer 304, and the metal layer 304 corresponding to a nozzle plate 360 is thus obtained, as shown in FIG. 12B. Nozzles 351 each having an inner wall with a substantially linear shaped cross-section (straight shape) are formed in the nozzle plate 360.
Japanese Patent Application Publication No. 10-296982 discloses a third related method of manufacturing a nozzle plate based on an electroforming method. Firstly, an opaque metal film is patterned onto a transparent substrate, and a photosensitive resist layer having a thickness of 100 μm made of a photocurable resin is formed on the opaque metal film. Thereupon, the resist layer is exposed to light via the opaque metal film from the side of the transparent substrate. In this exposure process, the amount of exposure light received by the resist layer is adjusted in such a manner that a strong exposure is achieved on the transparent substrate side, and the amount of exposure light declines as it moves toward the opposite side from the transparent substrate. The development processing is carried out subsequently, and a sharp end-shaped (tapered) resist which narrows in the direction of irradiation is formed. Then, a metal layer is formed on the opaque metal film and the metal layer is then separated from the transparent substrate and the resist, and the metal layer corresponding to a nozzle plate is thus obtained. The surface of the nozzle plate corresponding to the ink droplet ejection side (ink ejection surface) is the surface of the metal layer opposite to the transparent substrate.
However, there are the following problems in the methods of manufacturing the nozzle plate based on the electroforming method in the related art.
In the first related method of manufacture, as shown in FIG. 11C, the nozzles 251 having a curved cross-section on the inner walls have to be arranged at a certain interval with respect to the adjacent nozzles in accordance with their shape, and hence there are limitations on the degree to which the density of the nozzles can be raised. Furthermore, the diameter W of the nozzles 251 is uneven due to variation in the metal layer 204 formed to a thickness greater than that of the resist 202 by electroforming, and there is a problem in that this unevenness is liable to affect the size and flight characteristics, such as the ejection speed, of the ink droplets ejected from the nozzles 251.
In the second related method of manufacture, when the patterned resist 302 is formed on the metal substrate 300, as shown in FIG. 13A, an unpatterned resist layer 306 is formed on the metal substrate 300 and the resist layer 306 is then exposed to light from the upper side. More specifically, ultraviolet light 310 is irradiated onto the resist layer 306 via a mask 308 formed with apertures 308a corresponding to the prescribed nozzle shape. In this exposure process, a portion of the ultraviolet light 310 irradiated onto the resist layer 306 may be dispersed, and it may be reflected by the metal substrate 300 on the under side of the resist layer 306. When the developing process is carried out in this case, a resist 306a on the side adjacent to the metal substrate 300 assumes a broadened shape as shown in FIG. 13B, rather than a straight shape such as the resist 302 shown in FIG. 12A. There is a decline in the dimensional accuracy of the nozzles formed in this case, and the flight characteristics of the ink droplets ejected from the nozzles deteriorate.
In the third related method of manufacture, light exposure is carried out by adjusting the amount of light irradiated onto a 100 μm-thick resist layer, in such a manner that the light intensity is lower on the side opposite to the transparent substrate than it is on the side adjacent to the transparent substrate. Hence, there is slight variation in the amount of exposure light, as well as slight variation during developing, which adversely affect the dimensional accuracy of the resist formed into a tapered shape, leading to poor accuracy in the overall dimensions of the nozzles.
Furthermore, the dimensional accuracy of the resist is generally good on the base side; however, in the third related method of manufacture, the surface forming the ink droplet ejection surface of the nozzle plate is the surface on the opposite side to the transparent substrate, which corresponds to the base. Thus, the ink droplet ejection sides of the nozzles are formed on the basis of the resist shape that has inferior dimensional accuracy compared to the opposite side (the side of the transparent substrate). Therefore, there is a problem in that the dimensional accuracy of the nozzles on the ink droplet ejection side is not good, and this poor accuracy is liable to affect the ejection volume and the flight characteristics, such as the ejection speed, of the ink droplets ejected from the nozzles.